a2-adrenergic receptors (a2-AR) are cell surface receptors for catecholamines that bind to the Gi/G0 family of G proteins. a2-ARs are widely distributed in the central and peripheral nervous system, and are known to play an important role in the regulation of catecholamine release. This mechanism and the broad distribution of these receptors explain their role in a wide variety of physiological functions. a2-ARs mediate central hypotensive, sedative, anesthetic, and analgesic responses to a2-AR agonists. However, cardiovascular and other responses to a2-AR agonists are subject to interindividual variation in the human population. Such variability may be explained by genetic variation in the structure of the receptors themselves, the cognate G proteins, the transductional effectors, or the downstream intracellular targets. Molecular and pharmacological research has defined three a2-ARs subtypes designated a2A, a2B, and a2C. All three a2-AR subtypes are involved in the regulation of blood pressure, and these receptors also modulate sedation, analgesia, regulation of insulin release, renal function, cognition and behavior. Biochemical research has identified three human genes that uniquely encode these a2-ARs. Recently, in preclinical studies polymorphisms of all three a2-AR subtypes have been described. The three polymorphisms are each relatively abundant, and two appear to be functional in vitro. However, in humans the in vivo physiological effect of these polymorphisms is unknown. This study will elucidate the potential functional role of the three a2-AR subtypes in humans by comparing the behavioral, biochemical, psychophysiological, and autonomic function effects of the well-established a2-AR agonists and antagonists, clonidine and yohimbine, respectively, in individuals selected for particular a2-AR genotypes. Based on preclinical studies the following hypotheses will be tested: 1) subjects homozygous or heterozygous for the a2A-AR Asn251Lys substitution will show a potentiation of clonidine-induced effects, relative to subjects who have the Asn251/Asn251 genotype, and a reduction of yohimbine-indiced effects, 2) subjects homozygous or heterozygous for a a2B-AR three glutamic acid deletion (residues 301-303) will show reduced effects of the a2-AR agonist clonidine and possibly a potentiation of effects of yohimbine, and 3) we will evaluate whether altered responses in either direction occur in subjects homozygous and heterozygous for an in-frame deletion of a a2C-AR homologous repeat occurring at codons 322-325 relative to subjects without this deletion allele. We have recruited about 150 subjects for this protocol, and have assessed their genotypes and clinical and demographic information. In addition, we have performed yohimbine challenge procedures in 23 subjects but did not analyze the data yet, mainly because investigators are still blinded to the genotype information.